Wallets are well known. Traditional folding wallets of the prior art, however, are typically unduly large, thick, heavy and awkward to use.
“Card case” style wallets with a money carrying feature (i.e., a money clip, either metal or magnetic, or an elastic band-containing wallet) are also known, but entail their own set of problems:
Metal clips don't allow for dramatic fluctuation of bill quantities. Thus, one cannot put in 30+ bills easily and then try to go down to only 1 bill (because the metal is generally flexed in such a way that the tension no longer holds a single bill securely). Much like a paper clip that has been flexed outward on a large stack of paper, if one tries to then secure just two sheets, one has to bend the paper clip back to its original state to get enough tension. Paper clips however allow one to bend past the 0 degrees to get the tension correct. The metal in money clips, however, generally doesn't allow one to bend past 0 degrees, so one is left with a permanent gap unless one uses pliers or a vise on the edge to pinch the metal back down. Also metal clips are structurally bulky regardless of the amount of items being held in the clip. Metal clips also tend to wear through materials like pocket linings and pans materials. Even when wrapped in leather, metal clips tend to cut through clothing materials.
Magnetic clips (the kind where two magnets are sandwiched between pieces of leather or fabric and folded on itself via a “hinge”) are also generally bulky regardless of the amount of items being held within. Also, the magnetic options have a fault in that the more bills one folds underneath, the less the magnets are able to hold, since the distance between the magnets is being placed further and further apart with each added bill.